Pulse tube refrigeration process



March 11, 1969 1', J, W T EI'AL 3,431,746

PULSE TUBE REFRIGERATION PROCESS Filed Feb. 14. 1967 FIG.|.

POSITION urn nr FIG.4

United States Patent 3,431,746 PULSE TUBE REFRIGERATION PROCESS Thomas J. Webster, Ashford, and Michael E. Garrett, Addlestone, England, assignors to The British Oxygen Company Limited, a British company Filed Feb. 14, 1967, Ser. No. 615,972 Claims priority, application Great Britain, Feb. 21, 1966,

7,543/ 66 US. CI. 62-86 Int. Cl. F25b 9/00 2 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a pulse tube refrigeration process for cooling gases.

In such a process the gas to be cooled is compressed, passed through a regenerator where it is cooled, and then fed into the inlet of a pulse tube after turbulence in the gas stream has been substantially eliminated. The gas is compressed ad-iabatically towards the far end of the tube so that the gas becomes progressively warmer as it passes along the tube. The gas at the far end of the tube (hereinafter called the warm end) which has become heated as a result of compression in the tube is cooled by a heat exchanger, and then allowed to expand adiabatically back along the tube to the inlet (hereinafter called the 0001 end). This adiabatic expansion causes the gas to cool to a temperature which at the cold end of the tube is lower than the temperature at which the gas entered the tube. The cooled expanded gas then gives up some of its cold to a heat exchanger which enables the cold to be extracted ready for useful work. Finally the gas is returned over or through the regenerator which stores excess cold from the returning gas ready to pre-cool the gas entering during the next operational cycle.

B-y arranging for the pressure oscillations to occur at high frequency, a hundred times per minute or more, a comparatively large quantity of refrigeration can be obtained from a comparatively small volume of working fluid.

These processes have been operated at oscillation frequencies up to 400 cycles per minute; and it has been found possible to cool air initially at ambient temperature down to temperatures below 40 C. The quantity of refrigeration of the process increases in proportion to the oscillation frequency, but it has been found that one of the factors limiting the maximum frequency is the time taken to remove heat from the compressed gas at the warm end of the tube.

It is an object of the present invention to provide a pulse tube refrigeration process in which heat removal at the warm end is effected more expeditiously than hitherto, thereby permitting a higher frequency of oscillation, and in consequence a greater quantity of refrigeration from a unit of a given size.

According to one aspect of the present invention a pulse tube refrigeration process comprises the step of displacing warm compressed gas from the warm end of the tube in each operational cycle with adjacent cooler gas.

ICC

According to another aspect of the present invention in a pulse tube refrigeration process warm compressed gas at the warm end of the tube is displaced from the tube in each operational cycle, cooled, and then recompressed and recycled.

This method of displacing the warmed compressed gas permits the operational frequency of the process to be raised and increases the temperature difference between the gas entering and leaving the tube. The greater the operational frequency, the larger is the volume of gas which can be handled in a given time period, with a corresponding increase in the amount of refrigeration which can be produced in a given period of time.

The invention will be particularly described with reference to the accompanying drawing in which:

FIGURE 1 is a block schematic diagram of a known pulse tube refrigerator assembly,

FIGURE 2 is a temperature-position graph of the gas flowing through the refrigerator assembly of FIGURE 1,

FIGURE 3 is a block schematic diagram of one form of pulse tube refrigerator assembly for carrying the invention into effect, and

FIGURE 4 is a block schematic diagram of an alternative form of pulse tube refrigerator assembly.

The fundamental operational principles of a pulse tube refrigerator will now be described with reference to FIG- URES l and2.

The pulse tube refrigerator assembly comprises a two way valve 1- controlling the flow of compressed gas to and from a regenerator 2 connected through a heat exchanger 3 which also acts as a flow smoothing device to one end of a pulse tube 4. The other end of the tube 4 is connected to a heat exchanger 5, through which a suitable coolant, such as water may be passed.

In operation, compressed gas admitted through valve 1 is cooled to a temperature Tc as it passes from the warm end to the cold end of the regenerator 2. The gas then passes through the smoothing device 3 to reduce turbulence, and into the cool end of the pulse tube 4 where it compresses the gas already present therein thereby establishing a temperature gradient which attains a maximum at the warm end of the tube. This compression is basically an adiabatic compression which causes the compressed gas at the Warm end of the tube to heat up to a temperature T This gas at temperature T passes into the heat exchanger 5 where it is cooled to a temperature T and the gas in then allowed to expand adiabatically back along the tube 4 to the cool end thereby cooling to a temperature Te.

The returning gas at the cool end of the tube 4 then gives up some of its cold to the heat exchanger 3, and in so doing is warmed to a temperature Tc. This cold is extracted from the heat exchanger 3 by any suitable known means, and is held ready for useful work. One great advantage of the pulse tube refrigerator assembly is that it can produce cold at very low temperatures.

Finally the gas at temperature Tc is returned over or through the regenerator 2 which it cools in readiness for the next operational cycle.

One method of reducing the time taken to cool the gas at the warm end will now be described with reference to FIGURE 3.

In the assembly shown in FIGURE 3, the regenerator 2, heat exchanger and flow smoothing device 3 and pulse tube 4 are as already described with reference to FIG- URE 1.

Gas is admitted to the assembly through valve 6 and exhausted from the assembly through valve 7. At the warm end of the pulse tube 4 there is provided a valve 8 through which warm gas is discharged on completion of compression.

In operation, with valves 7 and 8 closed, valve 6 is opened to admit compressed gas. When the gas in tube 4 has been compressed, valve 8 is opened and the pocket of warm gas at the warm end of tube 4 is displaced at constant pressure, valve 6 remaining open. Valve 8 and valve 6 are then closed and valve 7 opened to permit discharge of gas returned through the regenerator 2. The sequence of valve operations is then repeated cyclically. It will be appreciated that the gas escaping from the warm end of tube 4 through valve 8 is lost to atmosphere. If the gas is disposable such as air, such loss can be tolerated, but if the gas is expensive, such as helium, then it is necessary to provide a closed circuit system as shown in FIG- URE 4. This circuit is also more eflicient in the production of refrigeration than that of FIGURE 3.

In the system shown in FIGURE 4 the regenerator 2, heat exchanger and flow smoothing device 3 and pulse tube 4 are as previously described with reference to FIG- URE 3.

Gas is compressed in a compression cylinder 9 and passes through a cooler 10 to the regenerator 2. A pressure-sensitive valve 11 is provided at the warm end of the tube 4, this valve 11 being arranged to open when the gas pressure at the warm end rises above a preselected level, and to close when the pressure falls below said preselected level. The gas which is expelled through the valve 11 is passed to a reservoir 12 where it is cooled before being passed through a non-return valve 13 to the compression cylinder 9 on its expansion stroke. The reservoir 12 is sufiiciently large to contain the gas expelled through the valve 11 during a number of cycles thereby allowing sufiicient time for the contained gas to cool be fore it is returned to the compression cylinder 9.

In operation, the piston in compression cylinder 9 moves upwards compressing the gas in the regenerator 2 and pulse tube 4. Near the end of the piston stroke the pressure in the pulse tube 4 causes the valve 11 to open and the warm gas is displaced at constant pressure from the pulse tube 4, expanding through valve 11 into the reservoir 12.

As the piston commences the return expansion stroke the pressure in the pulse tube 4 falls and valve 11 closes, the gas in the pulse tube 4 and regenerator 2 then expanding adiabatically against the piston. The gas at the cool end of the tube 4 then gives up some of its cold to the heat exchanger 3 and is warmed in so doing. This cold is extracted from the heat exchanger 3 by any suitable known means and is held ready for useful work.

When the piston is near the end of its expansion stroke the pressure is sufficiently low to allow the cooled gas to be drawn from the reservoir 12 via non-return valve 13 into the compression cylinder 9 to make up the original volume.

This cycle of operations is repeated with each cycle of the compression cylinder piston.

.We claim:

1. A pulse tube refrigeration process in which in each operation cycle:

(a) an inlet valve is opened to cause compressed gas to pass through a regenerator and a flow smooth ing device into a gas-filled pulse tube where it compresses the gas already present therein and establishes a temperature gradient which attains a maximum at the warm end of the tube,

(b) a discharge valve is opened to allow compressed gas to be displaced at constant pressure, from the warm end of the tube by adjacent cooler gas, and

(c) the inlet and discharge valves are closed and an outlet valve opened to permit discharge of gas returned through the regenerator.

2. A pulse tube refrigeration process in which in each operational cycle:

(a) gas is compressed in a compression cylinder and passed through a cooler, a regenerator and a flow smoothing device into a gas-filled pulse tube where it compresses the ga already present therein and establishes a temperature gradient which attains a maximum at the warm end of the tube,

(b) a pressure-sensitive valve is opened to allow only gas above a preselected pressure to be passed from the warm end of the tube to a reservoir for cooling, and

(c) the cooled gas from the reservoir is then passed to the compression cylinder on its expansion stroke.

References Cited UNITED STATES PATENTS 1,275,507 8/1918 Vuilleumier 6286 1,321,343 11/1919 Vuilleumier 6288 1,459,270 6/ 1923 Vuilleumier 6288 3,237,421 3/1966 Gifford 6286 3,302,422 2/1967 Smith 6286 3,314,244 4/1967 Green 6286 WILLIAM J. WYE, Primary Examiner.

U.S. Cl. X.R. 626 

